HMG-CoA reductase (HMGR) is a key enzyme of sterol pathway. HMGR is an integral membrane ER protein and is subject regulated destruction is mediated by ER-associated degradation (ERAD). Our initial discovery that HMGR regulated degradation is conserved in yeast has allowed using the uniquely facile approaches to unravel the mechanisms of HMGR ERAD and its regulation by the sterol pathway. The yeast HMGR isozyme Hmg2 undergoes ubiquitin-mediated ER degradation by the HRD quality control pathway. HRD- dependent Hmg2 degradation is controlled by levels of the sterol pathway molecule GGPP: elevated GGPP leads to increased entry into the HRD degradation pathway. We have made substantial progress towards understanding how the HRD machinery recognizes Hmg2 and other substrates, and how the sterol pathway controls entry of Hmg2 into the HRD quality control pathway. We have found remarkable similarities between the yeast and mammalian systems, including the nature of the signals, the use of ERAD as the degradative mediator, and the involvement of conserved motifs, in allowing regulation. Using the unique tools we have developed, we will push forward our parallel paths of study on HRD mechanisms and sterol pathway signaling. We will 1) Continue our study of the HRD E3 ligase complex, focusing on understanding the mechanism of misfolded membrane protein detection, and HRD complex regulation, 2) Analyze the mechanism of Hmg2 retrotranslocation from the ER membrane using a new in vitro assay developed by our group, in conjunction with genetic and proteomic approaches to discern the participating molecules in this still- mysterious process, 3) Study the features of Hmg2 allowing regulation by sterol pathway signals ? with a particular emphasis on the highly conserved sterol sensing domain (SSD) of Hmg2; and 4) Discover the nature and action of the sterol pathway signals that control Hmg2 ERAD testing the hypothesis that GGPP causes Hmg2 to undergo ligand- regulated misfolding to trigger HRD pathway entry. These studies provide the double benefit of understanding sterol regulation as well as proteostasis, both of great basic and biomedical interest.